The present invention is related to power amplifiers. In particular, the present invention is related to a predistortion circuit for linearizing power amplifiers.
With wireless communications, cable or Community Antenna Television (CATV), fiber optic, and related systems proliferating with great speed, significant related increases in associated multicarrier or other conventional applications for power amplifiers has increased. Along with the rise in such systems has been a corresponding increase in demand for efficient and linear power amplifiers for powering such systems including associated transmitters.
One basic problem in power amplifier design is the behavior of an operating power amplifier at amplitude and phase saturation levels. At such power demand levels which require power delivery at levels close to saturation, power amplifiers may experience drastic change in amplitude and phase characteristics, increasing the intermodulation (IM) distortion. It is generally desirable for power amplifiers to produce more output power and operate at higher levels of efficiency for a given level of distortion. Distortion generally arises from non-linearities associated with the transfer characteristics of the active or pass elements in the amplifier at signal frequencies. While it is possible to reduce distortion by operating amplifiers within a smaller, more linear range representing only a portion of the possible power output, it is often desirable to operate at the maximum possible output power to increase operating efficiency.
One solution to known distortion characteristics associated with an amplifier which is going to be operated outside its linear range, is to "predistort" the signal to compensate for known distortion characteristics. Such predistortion however gives rise to accompanying amplitude and phase distortion, envelope delay and the like. Many such predistorters are known.
A predistorter may generally be either of transmission or reflection type and may generally comprise a linear and a non-linear branch. By combining signals propagating through the linear and non-linear branches, both amplitude and phase variations of a power amplifier may be corrected. One such solution is suggested in the French patent application FR-2684820.
The reflective predistortion linearizer 100 described therein includes a linear and a non-linear branch, for example, as may be shown, for example, in FIG. 1. The linear branch may contain inductor 110 coupled to ground 102 which produces a reflected wave proportional to the amplitude of the incident wave input at input node 101. The second branch may include attenuator 130 together with two antiparallel schottky diodes 121 and 122 connected to ground 102, thus producing a reflective wave at combiner 140, which produces amplitude and phase deviations at higher powers due the non-linearity of the diode-resistance. The two reflective waves may be combined vectorially at combiner 140 to produce the linearizer's response and a corresponding compensated signal output at output node 141.
Problems arise however with the abovementioned linearizer. One problem is that amplitude and phase characteristics cannot be change to adapt to changing signal conditions. Another problem is that antiparallel diodes which are common in prior art linearizers are difficult to accurately bias. Still another problem is that isolators or matching networks must be used for favorable impedance matching between the amplifier circuit and the linearizer giving rise to the possibility of even more distortion and the potential need to compensate for additional transfer characteristics.
Another reflective device may be found in the prior art described in U.S. Pat. No. 5,146,177 to Katz, et al. Therein a balanced reflective nonlinear processor is described using FETs. Active FET devices are coupled to hybrid coupler 14 to perform nonlinear modification of signals. A problem arises however, in that active FET devices are more complicated to produce and operate. Katz, in fact, acknowledges the simplicity, cost, and reliability advantages of simpler solutions and further acknowledges that conventional passive solutions require additional impedance matching circuits.
Thus, it can be seen that while certain distortion problems may be addressed by the use of conventional predistorters, the difficulty posed by, for example, impedance mismatching and lack of adaptability remains inadequately addressed. It would therefore be appreciated in the art for an adaptive predistorter capable of compensating for non-linearity while avoiding the need for additional impedance matching.